A Free Electron Laser (FEL) is a form of laser device invented by J. M. J. Madey in 1971. The FEL is a laser device in which the optical gain is provided by the motion of free electrons, as opposed to electrons that are bound to lasant ions, or are moving about in a semiconductor. In a typical FEL device, a laser beam propagates coaxially with a relativistic electron beam, or e-beam, in a resonator. The e-beam is generally a very high-energy beam, which is usually produced by an accelerator. The e-beam typically passes through a transverse periodic magnetic field produced by a regularly spaced array of magnets of alternating polarity. The magnet structure, known as an “undulator” or “wiggler magnet”, is designed to cause the e-beam to exchange energy with the transverse magnetic field. This energy interchange tends to generate a transverse oscillation in the e-beam at the same frequency as that of the laser beam, thereby amplifying the laser beam, and providing optical gain.
The FEL has characteristics that are particularly useful in certain types of laser applications. A typical FEL generates a very high quality output beam, and can be operated at very high power levels, as compared to most other types of lasers. The FEL can also be operated at virtually any wavelength, in contrast to other laser technologies. It is therefore a very flexible source of coherent radiation, and is an attractive device for Directed Energy and ladar applications.
As FEL devices are scaled up in power, however, the resonator end mirrors are typically subjected to very high power densities. In addition, matching the e-beam and laser resonator beam for highly efficient conversion can be a difficult process. To achieve strong beam interaction, and thereby high optical gain, it is generally desirable to confine both the e-beam and the laser beam as tightly as possible within the wiggler magnet gap. The use of a small diameter laser beam, however, typically leads to an undesirable amount of beam divergence over the long electron beam interaction path generally needed for efficient amplification of the optical beam, with a corresponding reduction in optical gain. Moreover, the use of a small diameter optical beam in an FEL generally results in very high power densities on the resonator mirrors that would typically limit the power capabilities of an FEL resonator with conventional mirrors. To mitigate this power limitation, so-called “grazing incidence” resonator mirrors have been developed, which are designed to spread the power density over a relatively large surface area. However, the grazing incidence type of mirror design is generally very costly, due to the complexity of the mirror fabrication. As such, it is desirable to configure a type of FEL laser with minimal divergence of the laser beam along its interaction path with the e-beam within the FEL resonator, and also to reduce the optical power density impinging on the resonator end mirrors.
Accordingly, it is desirable to provide an FEL configuration that enhances the interaction of laser beam and e-beam by confining the e-beam and the optical field as tightly as possible within a wiggler magnet gap. In addition, it is desirable to reduce the optical power density on the resonator end mirrors of an FEL, to avoid the added cost and complexity of using a grazing incidence or other special type of mirror structure. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.